1. Field of the Invention
This invention relates to a pattern forming process, an apparatus for preparing the pattern and a process for preparing a semiconductor device utilizing the pattern forming process, mope particularly to a process for forming a pattern by etching, etc. of a material to be worked of a semiconductor, a metal, an insulator, etc., particularly to a process forming an etching pattern suitable for a pattern formation by a fine etching, etc. such as pattern formation of an electronic device, etc., an apparatus for forming the pattern and a process for preparing a semiconductor device utilizing the pattern forming process.
2. Related Background Art
In recent years, with the enlargement of an aperture of an exposure device to be utilized in a pattern forming technique utilizing photolithographic techniques, a large area device which is seen in a flat panel such as an active matrix liquid crystal display, a direct current plasma display, etc. is going to be commercially produced.
The pattern forming process utilizing photolithography to be,used for the formation of various patterns which these device have includes the steps of coating the predetermined surface of a material to be worked with a photoresist, subjecting the photoresist to treatments including pattern exposure, development to form a resist pattern, and affecting an etching by using a solvent or utilizing a physical or chemical reaction in gas phase while using the resist pattern as a mask.
Whereas, in the case of a VLSI chip, since the chip is formed by preparing some ten or more chips on one wafer at the same time under an integrated state and cutting it by dicing, it is possible to select and exclude the defective chips on one wafer. In contrast, in the case of a large area device, the number of the chips taken out from one wafer is no more than 1 to several. Only by modifying the device design such as reduction of the total number of the resist masks necessary for pattern formation utilizing photolithography, optimization of the design rule, etc., it is difficult to improve a yield which is one of the primary factors for determining the device cost.
Thus, in the preparation steps of semiconductor devices, the photolithographic process of forming a device structure by applying a fine working to a sample substrate according to a desired pattern is one of important techniques. As described above, in the photolithographic process, complicated and cumbersome processes such as resist coating, pattern exposure, developing, etching, resist peel-off, etc. have been generally employed.
And, as described above, as represented by a semiconductor memory device in recent years, an enhancement of capacity and performance of the device have been rapidly progressed, whereby the circuit pattern is becoming finer and also the circuit structure further complicated. On the other hand, the display device such as a liquid crystal display, plasma display, etc. becomes increasingly enlarged, and also the device functions are going to be more complicated. Accordingly, when these devices are to be prepared according to the process as described above, due the more complicated process itself, the cost will be increased. Further, due to generation of dust, etc., the yield will be lowered. Also the cost will be increased as a whole.
As attempts for accomplishing a high yield in the pattern formation utilizing photolithography, two methods have been investigated.
Since a defect generation depends on the environment in the lines of film formation and pattern formation, one method is to effect a cleaning of the working steps in these lines.
The ultra-cleaning technique (see Nikkei Microdevice, Extra Vol. No. 2, October, 1986) has enabled the reduction of a defective generation density by two orders or less as compared with the prior art.
However, when these methods are employed, a great installation investment for auxiliary equipments, automatic conveying apparatus, etc. for holding the standards with respect to purity of pure water, gases, etc. to be used at an extremely high level is required.
The other method is a method to add the defect correcting step to the fine working process so as to provide a redundant circuit in the device (Branch of Society of Applied Electronic Physical Properties, Research Report No. 427, p. 13).
Whereas, in this method, the process of photolithography becomes complicated, and also the problem remains that the cost and time required for inspection and correction increase.
In view of the state of the pattern formation technique utilizing photolithography as described above, it has been attempted to develop pattern formation techniques without using photolithography.
As one of them, there have been proposed the method utilizing the optical process (see "Examination Research Report Concerning New Electronic Materials XIII" 62-M-273, Society of Electronic Industry Promotion of Japan). Particularly, a photoetching is a process which effects an etching at the photoirradiated portion by a site selective photoirradiation to a material to be worked in the vacuum capable of maintaining a clean environment. None of the steps of the coating of the resist pattern, the pattern exposure, or the developing are utilized. Thereby, a high yield can be expected, there is little ionic shock, and also a great cost-down is possible by reducing the materials cost necessary for formating the resist pattern in photolithography. Thus, this process attracts attention as an ideal pattern forming process.
As one example of the photoetching processes, there has been known a process in which, by selectively irradiating an excimer laser to a polycrystalline silicon substrate added with phosphorus in an atmosphere where chlorine gas and methyl methacrylate are permitted to coexist, a polymerized film at the non-irradiated portion and the portion irradiated with a small irradiation dose is formed, while the polymerized film is not formed at the portion irradiated with a large irradiation dose, and then the substrate is etched by chlorine radicals to form a pattern (SEMI Technol. Symp. 1985, P. F-3-1).
Further, a photoetching technique which performs the formation of a pattern shortening the cumbersome process to a great extent in place of the photolithographic process by use of the resist as described above is also described in Sekine, Okano, Horiike: The Fifth Dry Process Symposium Lecture Pretext, p. 97 (1983). In this essay, there is reported the process in which a substrate deposited with a polysilicon (p-Si) film thereon is set within the reaction chamber where chlorine gas is introduced, a UV-ray is selectively irradiated through a photomask to the Si substrate by setting the photomask having a light shielding pattern in parallel to the substrate surface, whereby an etching proceeds only at the region where the UV-ray is irradiated to form a pattern on the p-Si film. By use of the phototreatment apparatus utilizing the process, the steps of resist coating, developing, resist peel-off, etc. can be obviated to simplify the steps, improve the yield and reduce the cost to a great extent. Further, etching without the generation of damage by ion irradiation which is a problem in the reactive ion etching of the prior art is rendered possible.
Nevertheless, the photoetching process as described above still has various problems to be solved.
For example, as compared with the electron impingement dissociation area of the molecules in the plasma in the dry etching of the prior art, the light absorbing sectional area of the molecules by photons is smaller by about 1 to 2 orders, and therefore the amount of the radicals which are reactive molecules or atoms having effective non-bonding arms necessary for etching is small. For this reason, it is required to increase the irradiation dose.
However, by the photoirradiation apparatus presently available, a sufficiently satisfactory irradiation dose cannot be obtained, and photoirradiation for a long time will be required.
For example, J. Vac. Sci. Technol., B3 (5), 1507 (1985) discloses the acceleration of etching by use of an excimer laser which is a large output laser. But the irradiation area in this method is about some ten mm square. For example, when the treatment area is about A4 size, the time required for the etching treatment will be one hour or more.
For obtaining 8 sufficient irradiation dose in the photoetching, the realization of enlarging a laser aperture and a laser output must be awaited, and it is difficult under the present situation to apply photoetching to the fine working process for the sheet system high speed etching accompanied with the enlargement of the device.
As a process for improving such problems in the photoetching, the process in which a high speed etching is performed by combining the photoetching with the plasma etching has been proposed.
This process comprises selectively irradiating an excimer laser to the surface of a material to be worked by etching while decomposing a gas of a hydrocarbon compound of which a part or all of the hydrogens is substituted with halogen atoms to deposit a polymerized film thereon, decomposing and removing the polymerized film at the photoirradiated portion, and further etching the surface of the material to be worked consequently exposed by halogen atom radicals, thereby obtaining an etching pattern corresponding to the photoirradiated pattern (Japanese Laid-open Patent Application No. 62-219525).
However, even in such a process, the photoirradiated area is as narrow as several ten mm square, and the etching speed is about some 1000 .ANG./min, the improvement of one order or more as compared with the photoetching as described above is not desired, and it could be unsatisfactory to apply to the fine working process for a sheet system high speed etching accompanied with the enlargement of the device.
Further, according to such a process, there is also the problem that the material to be worked may be damaged by the shock of halogen element radicals against the material to be worked.
Further, in the phototreatment apparatus as described above, when the photoetching is carried by selectively varying light intensities, it is necessary to perform the photoetching at plural times by exchanging the photomask to be used. As an example, the steps when preparing an Si image sensor constituted of laminated films is explained below.
For the laminated product having a Cr electrode 162, an SiN film 163, an Si film 164, an n.sup.+ -Si substrate 161 as shown in FIG. 1, it is necessary to have the first step of removing the unnecessary portion of the ohmic layer (n.sup.+ -Si film 165) during a channel formation to make it the state as shown in FIG. 2, and the second step of removing the unnecessary portions of a semiconductor layer (SiN film 163, Si film 164 and n.sup.+ -Si film 165) to form the state as shown in FIG. 3. In these respective steps, it is necessary to carry out the photoetching by use of photomasks having patterns different from one another.
Accordingly, in the phototreatment apparatus of the prior art as described above, when selectively performing the photoetching with different intensities, there is the problem that a phototreatment etching is required to be performed by exchanging the photomask every time, whereby the preparation steps are complicated.
In addition, in the pattern forming apparatus by use of the selectively irradiated light by the above-mentioned photoetching technique, a part of UV-ray light transmitted through the photomask is reflected at the surface of a sample substrate, and further a part of the reflected light is reflected at a light-shielding pattern surface of the photomask to be returned to the above sample substrate and to irradiate a non-irradiated region of the sample substrate, whereby there is the problem that the predetermined pattern cannot be often formed with a good precision on the sample substrate.
As described above, the main preparation steps of semiconductor devices are the steps of forming a film of metal, semiconductor or insulator on the substrate and fine working the film to a desired pattern. Presently, in the film forming step and the etching step of performing the fine working for the preparation of these semiconductor devices, the main step comprises, rather than the step using a solution, the so-called dry step using a plasma or excited gas in vacuum or reduced pressure gas. However, for carrying out a desired fine working, in the photolithographic process generally employed, as described above, the complicated processes such as resist coating, pattern exposure, development, etching, resist peel-off, etc. have been employed. However, since a solution is used in the resist coating, developing, resist peel-off steps, all the steps cannot be made a dry step.
Since in the semiconductor preparation steps as described above the steps of performing in vacuum, in solution and in air are mixed therein, the surface of the samples will be oxidized and also the preliminary steps for the subsequent steps are required (e.g. steps of vacuuming for making the sample under a reduced state, washing and drying after a solution treatment), whereby there is the problem that the number of steps may be increased and the steps become complicated. Also, the movement amount of the samples between the respective preparation apparatus will be increased, whereby losses in both time and space are increased. The increase of step number, complicatedness of the steps, increase of movement amount of samples involve the problem of increasing the attachment of dust. Further, by use of a resist and peel-off thereof, the resist peeled off becomes dust to be attached on the sample surface, whereby there is the problem that performances of the device deteriorate and also the yield is lowered.